No, one of the most unattractive things to me. I like guys who are naturally thin (skinny, really). Being fat isn't just about an unhealthy lifestyle, it's a body type generally so even if he was really fit instead he'd still have a different build from guys who're my type.
I'll tell you why I'd give that question the answer of a soft no. Losing a few pounds, no one cares, of course, but someone who has taken on the challenge of losing a lot of weight, hundred's perhaps, has to go on this huge roller coaster of emotions and stress and fighting setbacks and trying to change habits and often times, that level of weight loss becomes their everything until they lose it. I wouldn't necessarily want to get involved in that stage of their life because like an alcoholic in recovery, they are desperately trying to rid themselves of often times a life long addiction to food and in many cases, complete lack of exercise. Besides, a lot of people when they go through something like that keep their supporter at their side during their struggles, but as soon as they think they look all hot and good and healthy again, they drop them simply because they feel or do have more opportunities to date around that perhaps weren't available to them. I did some deep thinking on this one, lol.